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Characterization of Variable Platform for Robust Sensor and Separation
Nanocomposite Membranes
1
Olson ,
J. M. W.
Y.
W. R.
1Chemical & Materials Engineering, San José State University, 2SRI International
Objectives
2
Blum ,
Materials
The goal of this research project is to create and characterize nanocomposite membranes using
nanoparticles andsiloxane-based. Siloxane-based polymers have already been already reported as
selective membranes for CO2. The chosen nanoparticles are nano graphene platelets that were recently
discovered and are considered a low cost replacement of carbon nanotubes for enhancing mechanical
properties of nanocomposite materials as well as electrical conductivity. Such composite membranes
hold potential to produce a viable gas/electrical sensing membrane. Hence the creation and
characterization of such materials is of interest.
Results
The siloxanes selected for this study are polydimethylsiloxane (PDMS), a
polymer with numerous commercial applications, and modified
polyhydromethylsiloxane (polyhydridomethylsiloxane, PHMS), based on a
polymer platform developed by SRI International with great chemical
diversity. Both polymers possess high thermal resistance and high CO2 selectivity. As a result of their molecular
composition, they differ in their polarity and cross-linking characteristics. The conventional PDMS is non-polar,
lightly crosslinked, and elastomeric in nature. Replacing one methyl group with a different functional or
crosslinking group (e.g. modified PHMS, PRMS) can change the affinity and degree of crosslinking.
Theory
There are several mechanisms proposed to explain why the addition of a dispersed phase within a
polymer matrix would enhance the gas permeability, selectivity, and sensing abilities of the material:
● Nanogaps – the dispersed phase is not perfectly adhered to the matrix, resulting in nanogaps along
the interface through which gas might travel
● Polymer chain packing disruption - increases the free volume through which gases as passed
● Chemical modification – chemical interactions between the dispersed phase and the diffusing material
Transport of large molecules is diffusion limited, while transport of small molecules is absorption limited.
Therefore, mechanisms which increase diffusion, while not affecting absorption, may increase selectivity
and sensing of larger molecules.
In order to create the most efficient sensitivity with the least amount of material, the path the gas
molecules must travel should be as long as possible. This will also increase the selectivity. To achieve this,
a tortuous path is created, as in the figure below, left. The best possible shape for the dispersed phase is
thus a flake, oriented perpendicular to the net path of gas flow. To this end, pre-exfoliated graphene has
been selected as the dispersed phase to be placed in various crosslinked siloxane matrices.
Adapted from: B.Z. Jang and W.C. Huang, “Nano-scaled graphene plates,” US Patent 7071258
(October 21, 2002).
The structure of graphene,
above, is rather like an
unrolled carbon nanotube.
Graphene shares many of the
properties of carbon
nanotubes like high thermal
and electrical conductivity and
strength. In fact, its shape
may make it superior to
nanotubes for the studied
application.
Lee, D. Cho, and L.T. Drzal, “Real-time observation of the expansion
behavior of intercalated graphite flake,” J. Mater. Sci., 40, 231-234
(2005).
It is important that the dispersed
phase be exfoliated, as illustrated
at right. This will result in
maximization of the surface area
for possible chemical reaction.
Hay, J.N. and S.J. Shaw (2000). Clay-Based Composites [Online]. Available at
http://www.azom.com/details.asp?ArticleID=936 (accessed September 21, 2007). WWW Article.
Processing
The process begins with
using borosilicate glass
filters as the backbone of
the membrane.
1
Chung
Since the graphene flakes are
larger than the pores in these
filters, any composite placed on
top of them will be graphite
enriched as the polymer
permeates the glass fibers. These
filters were thus soaked in PRMS
and cured.
K. Kalaitzidou, H. Fukushima, and L.T. Drzal, “Mechanical properties and
morphological characterization of exfoliated graphite-polypropylene
nanocomposites,” Compos. Part A, 38, 1675-1682 (2007).
Lee, D. Cho, and L.T. Drzal, “Real-time observation of the expansion behavior of
intercalated graphite flake,” J. Mater. Sci., 40, 231-234 (2005).
A mixture of solvent, liquid PRMS, and graphene
was created. The graphene made up 5% of the
volume of the composite (polymer and graphene
component of the mixture).
The process of producing nano-scale graphene plates
begins with a polymeric precursor. This precursor is then
carbonized (fully or partially). This results in an
intercalated graphite flake that looks like that pictured
top left. This single flake is made up of many, many
layers of graphene (middle left). Next, chemicals and
heat are added to exfoliate the layers. The flake will
expand from a thickness of 80-100 μm to a thickness of
up to 10 mm or more in three seconds. Fast digital
photography by Lee et al. captured this rapid
transformation (bottom left). The exfoliated graphite is
then ball-milled (or otherwise crushed) until nano-sized
graphene, as shown bottom far left, results. The
graphene flakes will consist of, as in the transmission
electron microscope image
at right, a small number of
layers with total thickness on
the nano-scale.
The graphene platelets used
in this project have an
average diameter of 35 μm,
with a maximum of 100 μm.
In thickness, at least 80%
are less than 100 nm.
Scanning Electron Microscopy (SEM)
was preformed on the PRMS/graphene
nanocomposites, the pre-coated filters,
and the raw filters. The Image at right
was taken of a raw glass filter. The
filter is clearly highly porous, and the
permeability of the composites can not
be determined by this mechanical
support. The image of the polymercoated filter, shown below, indicates
uniform coating throughout.
SEM on the composites was performed
on the surface and on a cross-section of
a specimen cut with scissors. The
composite appeared very rough, as
shown below right. The rough nature of
the composite increases the chance that
there are straight paths through the
composite for the gas. Also, literature
evidence suggests that platelets laying
perpendicular to the gas flow will be
most effective. Future processing will
focus on reducing this roughness.
Coated glass
filter
Nanocomposite
membrane
K. Kalaitzidou, H. Fukushima, and L.T. Drzal, “Mechanical properties and morphological
characterization of exfoliated graphite-polypropylene nanocomposites,” Compos. Part A, 38,
1675-1682 (2007).
The composite solution was applied
to the pre-coated filters and allowed
to cure.
Cross-sectional views of a specimen
(above) showed clear delineation
between the composite and the
supporting substrate. The composite
forms a nice, thick coating.
Individual graphene flakes could be
observed, as in right. It appears to be
the expected radius. These flakes are
so thin that structure beneath may be
seen in the image.
Gas Measurements
Future Work
The next step is to perform gas transport measurements and compare the
performance of the polymer-only filters with the composite membranes. This
will reveal if the roughness indicated by the SEM is providing fast paths
through the membranes. These gas measurements will first be done with
nitrogen gas and argon gas. Later carbon dioxide will be used, and possibly
other gases. At right is the test chamber that has been designed and
constructed for this purpose with automated computer data acquisition.
● Additional membranes will be created using smaller amounts of
graphene by volume.
● Additional membranes will also be made with modified PHMS and
PDMS.
● Process refinements will focus on creating flatter surfaces with flakes
aligned parallel to the substrate.
Acknowledgments
J. Olson is supported by Defense Microelectronics Activity Cooperative Agreement #H94003-07-2-0705.
Scanning electron microscope images were possible thanks to National Science Foundation grant
#0421562 for Major Research Instrumentation.
Graphene donated by Angston Materials, LLC.
Thanks to: Prof. E. Allen, Prof. M. McNeil, D. Hui, A. Micheals, T. Olson , N. Peters and D. Verbosky.

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